The elements of the modular assembly system are disclosed in U.S. Pat. No. 3,838,535 and comprise, among others, a wheel which is described in U.S. Pat. No. 4,376,351. The wheel has a square central axle bore with a cross-section of a.times.a and rectangular slots of the width a which are axially opposed in pairs and which are cut radially into both of the lateral faces of the wheel.
With the wheel according to U.S. Pat. No. 4,376,351, the plug-in-type elements of the system can only be plugged onto the wheel by means of these radial slots, i.e. by a non-positive connection. With higher speeds of the wheel the strength of this non-positive radial connection between wheel and plug-in-type element may be overcome by the considerably increasing centrifugal forces so that the connection will give way.
Furthermore, this kind of connection between the wheel and the system element is too specific and restricts the possible combinations, i.e. the usability of the wheel, in a very unsatisfactory way.